Small electronic hearing devices for being worn at an ear or within an ear canal of a user are becoming increasingly popular. Examples of such devices are earphones, for instance used in conjunction with personal audio/video players, gaming units and mobile phones, ear-level communication devices, active hearing protection devices, in-ear monitors as well as hearing aids, sometimes also referred to as hearing instruments or hearing prostheses. Such devices are available in a number of different form factors depending on how they are worn, for instance as behind-the-ear (BTE), in the crest of the cymba, in-the-ear (ITE), in-the-canal (ITC), completely-in-canal (CIC) or hybrid BTE/ITE devices. In many applications it is preferred that the device is as inconspicuous as possible, e.g. for reasons of aesthetics and wearing comfort. This is frequently achieved by placing the device into the ear canal of the user, either partly or fully. Alternatively, the devices are designed to be small enough to fit into the crest of the cymba or to be worn entirely behind the pinna.
In order to provide an audio signal to the ear drum of the user all the mentioned devices require a receiver, i.e. a unit that converts an electrical signal conveying the audio signal into acoustic energy in the form of sound waves (more generally referred to as an electro-acoustic transducer). Such receivers need to be very small, especially in order to fit into the ear canal of a person. Smaller receivers allow to design hearing devices which can be inserted deeper into the ear canal, e.g. into the bony portion, which provides the benefit of reduced occlusion effect. This particularly enables the design of deep-fitted CIC devices. Moreover, smaller receivers allow to design hearing devices that occlude the ear canal to a lesser extent, i.e. that leave the ear canal more open when the hearing device is inserted, providing the benefit of increased wearing comfort as well as a more natural sound perception. Especially open-fitted hybrid BTE/ITE devices, popularly referred to as receiver-in-canal (RIC) or receiver-in-the-ear (RITE) devices, benefit from such small receivers. Furthermore, also BTE devices and devices worn in the crest of the cymba profit from small receivers since it is mainly the size of the receiver that determines the degree of miniaturisation achievable for these devices.
An example of such a miniature receiver is disclosed in EP 0 851 710 A1. Such receivers typically have the shape of a rectangular cuboid, which is especially not optimal in view of the oval-shaped cross-section of the ear canal. Alternatively shaped receivers are shown in EP 1 209 948 A2. A receiver with reduced thickness is described in U.S. Pat. No. 5,960,093. In all of these publications the acoustic membrane, also called diaphragm, is arranged longitudinally within the receiver casing in order to maximise the size of the diaphragm and thus to achieve a high output sound pressure level. Contrary to this EP 0 548 580 A1 proposes a cylindrical receiver for in-the-ear applications where the diaphragm is arranged laterally with respect to the cylinder axis at an open end of the cylinder. However, this design constrains the size of the diaphragm to that of the ear canal cross-section which has the disadvantage of limiting the achievable output sound pressure level relative to the designs mentioned previously.
In addition, it can be beneficial to provide a microphone (i.e. an acousto-electric transducer) in the mentioned devices capable of picking up the sound present in an ear canal of the user of the hearing device. Such a microphone is referred to as an “ear canal microphone” in order to distinguish it from the one or more microphones used to pick up the sound from the surroundings of the hearing device user and therefore referred to as “ambient microphones”. Such ear canal microphones can be utilized for a number of purposes. For instance WO 02/17838 A1 discloses a hearing protection device comprising means for verifying proper functioning of the device in terms of sound attenuation, i.e. of sufficient acoustical sealing. The means include an ear canal microphone for measuring the sound present in the ear canal. Furthermore, EP 1640972 A1 proposes a system and method for separation of a user's voice from ambient sound. The system comprises a device to be worn at least partly in the user's ear canal with a first microphone oriented outwardly towards the environment and a second microphone oriented inwardly towards the user's ear canal. An audio signal processing unit processes the audio signals from the first and second microphone using a blind source separation algorithm adapted to separate the user's voice from ambient sound. Furthermore, US 2005/0058313 A1 provides an ear-level communication system comprising an ear mould housing a microphone for use in an ear canal, a processor, and a wireless transmitter. The microphone picks up the user's voice from within the ear canal which is then processed by the processor and transmitted to a remote receiver by means of the wireless transmitter. Furthermore, U.S. Pat. No. 5,987,146 describes an open ear canal hearing aid system which comprises a plurality of ear canal tubes sized for positioning in an ear canal of a user. A receiver and/or microphone are located in the ear canal at the end of ear canal tubes. Furthermore, in U.S. Pat. No. 6,937,738 a digital hearing aid comprising an occlusion subsystem is presented. The occlusion sub-system compensates for the amplification of the digital hearing aid user's own voice within the ear canal. To achieve this, an ear canal microphone captures the sound in the occluded ear canal and provides the signal to circuitry which electronically cancels or minimises its effect. US 2008/0181440 A1 presents a combined receiver and ear canal microphone assembly which for instance can be used in a digital hearing aid according to U.S. Pat. No. 6,937,738. For all these applications it is desirable that the employed ear canal microphones be as small as possible, especially in those instances where both a receiver and an ear canal microphone are to be used, as is the case for active occlusion cancellation mentioned in the last example above.